In the field of optical applications, GaN-based light emitting devices including light emitting diodes (LEDs) and laser diodes have attracted great attention in recent years because these devices are capable of generating short wavelength emissions in UV and blue regions, which can have many practical applications such as high density storage, high speed data processing, solid state lighting, flat panel color display, and quantum computing. The realization of GaN-based layers, however, is relatively recent in comparison to GaAs-based layers. Therefore, the technology of GaN-based layers is still in the development stage, and many technical issues remain to be addressed and resolved before those applications can be realized.
Considering the state of the art, it is known to produce GaN-on-sapphire templates for blue LED mass production. In a first step of a conventional approach, a GaN nucleation layer is grown on a sapphire substrate. In a second step, a two to four microns thick GaN buffer layer is grown on the nucleation layer. This growth step is very time-consuming and takes typically from two to four hours. In a last step, an InGaN/AlGaN/GaN-LED structure including cladding layers, multiple quantum valves and p-type layers with a total thickness of the LED structure of about 1 μm is grown on the GaN buffer layer.
Despite the fact that a high device yield can be achieved with such conventional technology, the resulting structures have some disadvantages. While the sapphire substrate is less expensive, and a more popular choice than a high cost GaN-substrate, it is non-conductive, requiring two wire bonds on top of each chip. With the electrical current travelling laterally between these two contacts, the packaging efficiency is greatly reduced. While sapphire is transparent, enabling more light to escape from the chip, it unfortunately acts as a thermal insulator that traps heat, dramatically reducing the high operating current efficiency and ultimately limiting the available applications.
Furthermore, due to the lattice mismatch and temperature expansion co-efficient difference between sapphire and GaN, the GaN device structures grown on a sapphire substrate are known to have many defects that tend to affect the device performance. Other factors, such as the insulating property and non-cleavage of sapphire material, make manufacture of a GaN light emitting device with such conventional process technology difficult.
Instead of sapphire substrates, SiC substrates can be used to grow thereon a GaN-layer. However, although conductive, SiC traps a substantial portion of the light being emitted because massive absorption occurs only in the UV range.
Therefore, in another known approach for producing vertical GaN-LEDs, under consideration of the above-mentioned advantages and disadvantages of sapphire and SiC substrates, a sapphire substrate can be used as the initial GaN growth substrate followed by bonding a thermally and electrically conductive metal layer on top of the GaN. By then employing an appropriate lift-off technique, the sapphire substrate is lifted off the GaN, leaving it and the reflective base ready for the fabrication of vertical devices.
The result of a vertical device being bonded to a reflective metal layer that exhibits low thermal resistance, and high electrical conductivity, leads to efficient devices that lend themselves to thinner LED packaging while remaining rugged enough to retain comfortability with traditional die-mount techniques. Due to a high brightness, this approach is especially advantageous for back light applications such as cellular phones, where a thinner die saves precious space, as well as for high power/super bright applications, such as solid-state white lighting.
Nevertheless, even this approach cannot prevent or avoid the disadvantages arising out of the difference of material properties between the sapphire substrate and the GaN-layer grown thereon. In particular, the dislocation density of the active nitride layer(s) of such substrates, which is usually in the order of 108/cm2, strongly restrains the efficiency of optical devices fabricated with such substrates.
It is therefore desired to provide a method of forming GaN-type layers in which the crystalline quality of the active nitride layer(s) can be improved.